1. Field of the Invention
The present invention relates to a pump device for supplying liquid toner to an electrostatic plotter which uses liquid toner to develop electrostatic latent images on paper sheets.
2. Description of the Prior Art
In the prior art, the duct for conveying liquid toner from a pump to a liquid-toner supplier unit is branched into two ducts, one of which is provided with an aspirator. A part of the liquid toner supplied by the pump is discharged through the aspirator. The flow of the liquid-toner through the aspirator generates negative pressure therein. The excess of the liquid toner supplied onto a printing paper sheet is sucked through a suction port unit into the aspirator by the negative pressure. The sucked toner is discharged together with the liquid toner which is flowing from the pump through the aspirator.
Since the aspirator must be supplied with a flow of liquid toner, as described above, the pump has to force out more liquid toner than actually required for printing; in some cases, the amount exceeds two times the actual requirement. Therefore, a large pump is required, which takes up a large space for installation and thus is economically disadvantageous.
Also, since there are limits in respect of the installation space or costs, a pump or an aspirator of sufficient size or capacity can not always be employed. An aspirator of insufficient capacity may fail to suck the excess liquid toner (not-adhered toner) present in creases or wrinkles on paper sheets even if they are small, so that the paper sheets may be stained and the images may be blurred.